1. Field of the Invention
The present invention relates to a deposition method that is used for forming a layer including an organic compound. Further, the present invention relates to a method for manufacturing a light-emitting device in which a layer including an organic compound is used as a light-emitting layer.
2. Description of the Related Art
A light-emitting element using an organic compound as a light-emitting body, which has features such as thinness, lightness, high-speed response, and DC drive at low voltage, is expected to be applied to a next-generation flat panel display. Among display devices, ones having light-emitting elements arranged in matrix are considered to be particularly superior to conventional liquid crystal display devices for their wide viewing angle and excellent visibility.
It is said that, as for a light-emitting mechanism of a light-emitting element, an EL layer is sandwiched between a pair of electrodes and voltage is applied to the EL layer, and thus electrons injected from a cathode and holes injected from an anode are recombined in an emission center of the EL layer to form molecular excitons, and the molecular excitons release energy when returning to a ground state; thus, light is emitted. An excited singlet state and an excited triplet state are known as an excited state, and it is believed that light can be emitted through either state.
An EL layer included in a light-emitting element includes at least a light-emitting layer. In addition, the EL layer can have a stacked structure including a hole injecting layer, a hole transporting layer, an electron transporting layer, an electron injecting layer, and/or the like, in addition to the light-emitting layer.
In addition, an EL material for forming an EL layer is broadly classified into a low molecular (monomer) material and a high molecular (polymer) material. In general, a low molecular material is often deposited by an evaporation method and a high molecular material is often used by an inkjet method, a spin coating method, or the like.
Reference 1 (Japanese Published Patent Application No. 2001-52864) and Reference 2 (Japanese Published Patent Application No. 2001-102170) disclose that an EL layer is formed by an ink-jet method.
In addition, a technique is disclosed in Reference 3 (Japanese Published Patent Application No. 2001-189192) in which an organic EL material is applied from a nozzle in a stripe manner, not in a dotted manner like in an ink-jet method.
In addition, a technique in which an organic EL material is applied by a method unlike an ink-jet method, that is, an application method with the use of a so-called nozzle printing device is also disclosed in Reference 4 (Japanese Published Patent Application No. 2002-75640).
Further, when a light-emitting element is formed using a high molecular material, there is a laser induced thermal imaging (also called LITI) disclosed in Reference 5 (Japanese Published Patent Application No. H10-208881).
In addition, when a light-emitting element is formed using a low molecular material, a method using a laser beam is disclosed in Reference 6 (Japanese Published Patent Application No. 2007-281159). A laser thermal transfer method disclosed in Reference 6 is also called laser induced pattern wise sublimation (LIPS).
By the ink-jet method disclosed in Reference 1 and Reference 2, a material which is to be deposited is dissolved or dispersed in a solvent or the like and liquid including the material is adjusted and prepared so that liquid can be discharged from the nozzle, and then the liquid is discharged from the nozzle, whereby deposition is performed on a deposition target substrate. By the ink-jet method, the amount of a droplet and the position of a droplet discharged from the nozzle are controlled, whereby deposition can be selectively performed on the deposition target substrate.
A disadvantage of the ink-jet method is that when a thin nozzle is clogged up for some reason and clogging occurs, a defective display panel having a dot defect or a line defect is formed.
Also, by the methods disclosed in Reference 3 and Reference 4, liquid including a material is prepared. The methods disclosed in Reference 3 and Reference 4 are superior to an ink-jet method in that there is a wide range of viscosity of a material liquid that can be used and in that a problem such that clogging due to drying of a nozzle tip or the like does not occur.
Note that a deposition target substrate will be part of a display panel later in any of References 1 to 4.